Easy, you have to weight the numbers.
A guy who plays in the SB is listed as 37 if he's one day before his 38th birthday, or 364 days before it. So Brady born August 3rd is actually 37.59 years old. Another guy listed at 37 might be 37.99 years old. Now I calculated that for all 53 players of all SB winning teams.
Next I weighted it. Take Jones, he played 34 minutes, while Gostkowski played 4. So Jones age is 34/60 of the age of the team, and Gostkowski is 4/60. Then I did that for all players of all SB teams.
Once all that was done I got 25.2 exactly.
And if you believe I did that I have a bridge for sale.
What they actually did was go to profootballreference- clicked on roster and read the number. For the Patriots 25.2.
http://www.pro-football-reference.com/teams/nwe/2014_roster.htm
It includes everyone who was on the team at anytime so around 75 guys. It's not necessarily the worst way, a guy that played 15 games but not the last wouldn't be in the SB but is arguably part of the winning team by getting them there. The numbers line up for what was reported as Seattles age last year as well (25.0). As for the 26.6 number. I don't know, but these reporters aren't exactly blowing me away with research so I'm sure they didn't calculate it. You can bet it's on another website somewhere whether right wrong, or a different methodology.